The invention is directed to a distributed optical fiber amplifier designed for soliton signal transmission. More particularly, the distributed optical amplifier is optimized to balance self phase modulation and linear dispersion as well as to minimize the effects of soliton--soliton interactions at very high bit rates over long, unregenerated distances.
A soliton is a light pulse which does not change in either the time or spectral wavelength domain when propagated in an ideal optical waveguide fiber. By ideal waveguide is meant, the waveguide is lossless and has a total dispersion, also termed, group velocity dispersion, which, together with the self phase modulation of the soliton, serves to maintain the invariance of a soliton. The non-linear self phase modulation is dependent upon the soliton intensity. Hence, a soliton is invariant only when its intensity is at the level required to balance total waveguide dispersion and self phase modulation. The soliton can still be capable of carrying information when the intensity level is different from that required for invariance, if the travel distance in the optical waveguide is not too great.
The introduction of optical amplifiers into telecommunications systems has made soliton data transmission practical. In the case of localized or lumped optical amplifiers, i.e., those where, for example, an erbium doped waveguide fiber is less than a few tens of meters in length, the spacing between amplifiers must be less than the soliton period. The soliton period is given by the relation, z.sub.o =9.53.times.10.sup.-5.times.T.sup.2 /S.sup.2 D, where Z.sub.o is the soliton period, T the soliton pulse width in ps, S the soliton wavelength in nm, and D the total dispersion in ps/nm-km. It is clear from the form of this relation, that the soliton period decreases with decreasing soliton pulse width. Thus, for systems requiring high data rates, which implies small soliton widths, the spacing of optical amplifiers becomes shorter. At 40 Gbps, the optical amplifier spacing must be less than 10 km, thereby placing a severe initial cost and maintenance burden on the system.
A potential solution to this spacing problem is provided by the distributed optical amplifier, wherein the dopant, e.g., erbium, is distributed along essentially the entire length of the waveguide fiber. By coupling a pre-selected pump light energy into the waveguide at appropriate length intervals, the waveguide fiber can be made lossless on a local basis so that the soliton intensity does not vary appreciably with length.
To achieve this condition of transmission of essentially invariant solitons using a distributed optical amplifier:
the attenuation of the pump signal in the waveguide must be taken into account; PA1 the concentration of erbium along the waveguide fiber must be chosen high enough to provide for lossless transmission and reasonable pump light efficiency; PA1 the concentration of erbium along the waveguide fiber must be low enough to keep the local gain low to provide for small excursions of soliton intensity.
The concept of distributed optical amplifiers is discussed in, "Performance of a Distributed Erbium-Doped Dispersion-Shifted Fiber Amplifier ", Simpson et al., Journal of Lightwave Technology, Vol. 9., No. 2, February 1991, and in "Loss-Compensation Characteristics and Noise Performance of Distributed Erbium-Doped Optical Fibers ", Wada et al., Electronics and Communications in Japan, Part 1, Vol. 75-B-I, Nov. 3, 1993. In each of these publications the necessity of keeping soliton energy deviations small is stressed. See Simpson et al., page 228, 2nd col., 2nd paragraph, "Future ultra-high bit rate systems will require . . . a transparent transmission line with only small excursions in the signal amplitude.", and, page 231, Summary, "Continued efforts to fabricate lower erbium concentration, lower loss fiber will . . . improve the performance of these distributed optical amplifiers". See Wada et al., page 75, 2nd col., 1st paragraph, "Especially in N=1 soliton propagation, to preserve the pulse shape along the propagation distance, it is necessary to keep the pulse-energy deviations below about 20%; and therefore optical signal transmission with small-level variations is required." Wada et al., further state the benefit of using bi-directional pumping to provide for longest compensated length (see Summary, page 75, 1st and 2nd paragraph).
In contrast to these publications, the present invention teaches different limits on soliton signal variation for systems of an intermediate length, for example, those case considered below in which the length is in the range of 50 km to 500 km. In addition, for these systems, the teaching of the invention contrasts with the literature with regard to the benefit of bi-directional pumping as compared to uni-directional pumping. This counter teaching stems from a careful evaluation of soliton--soliton interactions, over the intermediate system lengths, which can produce timing jitter of the soliton pulses and soliton collisions, i.e., collapse of two adjacent solitons into a single pulse.